European patent publication 431,648 of G. E. Keller (based on US application Ser. No. 07/447,996 filed Dec. 8, 1989now abandoned) describes a method for conditioning indoor air in a forced-air heating/cooling system by adding ozone to the stream of recirculated air to eliminate various pollutants, in particular volatile organic compounds (VOC's). Downstream of the location where the ozone is added, the air stream is passed through a catalytic filter arrangement to strip out the ozone before the air stream is reintroduced into a living space, which term is here intended to cover any enclosed area where people live, work, play, or are present for other purposes.
While ozone is an excellent material for oxidizing VOC's, it is itself a highly toxic substance whose maximum concentration in air should not exceed a threshold level of 0.1 ppm (MAK value). The concentration needed for effective decomposition of the various pollutants must exceed this threshold level by a great deal, so invariable a decomposition- or converter-type filter is provided downstream of the location where the ozone is introduced into the stream. This filter uses a catalyst, typically of platinum or nickel, to decompose the ozone (O.sub.3) into stable molecular oxygen (O.sub.2).
In the above-described system of Keller, two ozone sensors are provided, one upstream of the ozonator and one downstream of the decomposition filter. The upstream sensor determines the ozone content of the gas stream being treated before any treatment, and the downstream after it is ready to reintroduce into the living space. The outputs of these two sensors are fed to the ozonator and it feeds into the stream being treated an amount of ozone determined by them.
The main disadvantage of this system is that the amount of ozone added to the stream is normally calculated to be sufficient to take care of the maximum amount of pollutants that are expected to be encountered. This puts a considerable load on the decomposition filter, in particular when the actual level of pollutants is low so that most of the added ozone must be oxidized at the filter.
Another problem is that if a portion of the filter goes bad so that ozone can get through it without being oxidized, it is possible for a toxic level of ozone, that is a concentration exceeding the above-mentioned 0.5 ppm threshold, to be present in the air stream that is reintroduced into the living space. Clearly this should be sensed by the downstream ozone sensor, which is typically quite sensitive. Nonetheless practice has shown that since this downstream sensor only rarely is exposed to enough ozone to produce a meaningful output, it is fairly common for this element of the system to fail without anyone knowing that it has done so.